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Heifer Study: University of Missouri researchers recently compared the use of MGA and controlled internal drug release (CIDR) inserts and their impacts on heifer estrous response, timing of artificial insemination (AI) and pregnancy rates.

Eric Grant | Feb 01, 2006

Heifer Study: University of Missouri researchers recently compared the use of MGA and controlled internal drug release (CIDR) inserts and their impacts on heifer estrous response, timing of artificial insemination (AI) and pregnancy rates.

A total of 353 heifers at three locations were randomly assigned to one of two treatments by age and weight.

AI was performed at 72 hours after PG for cows assigned to the MGA Select treatment, and at 66 hours after PG administration for the CO-Synch + CIDR treatment. All cows were injected with GnRH at time of AI.

Three AI sires were used at location 1, and one at locations 2, 3 and 4. Cows were exposed to fertile bulls for natural service 14 days after AI for a 60-day natural service period at locations 1, 3 and 4, and for a 45-day natural service period at location 2.

Researchers found:

No differences between treatments at the respective locations for age, days postpartum, BCS or estrous cyclicity status at the initiation of treatment. However, there were differences among locations.

There was no effect of treatment, technician, or sire on pregnancy rates resulting from fixed-time AI.

This article was provided by the National Association of Animal Breeders. Learn more atwww.NAAB-CSS.orgor call 573/445-4406.

Table 1. Estrous response, AI pregnancy and final pregnancy rates

Estrous response

AI pregnancy rate

Final pregnancy rate

CIDR

154/177 (87%)

112/177 (63%)a

164/177 (93%)

MGA

147/175 (84%)

83/175 (47%)b

159/175 (91%)

Total

301/352 (86%)

195/352 (55%)

323/352 (92%)

Difference

+3%

a,bP = 0.01 +16%

+2%

Source: From Kojima et al. (2004)

Table 2. Pregnancy rates after fixed-time artificial insemination and at the end of the breeding season

Pregnancy rate to fixed-time AIa

Pregnancy rate at end of breeding seasonb

Item

Proportion

%

Proportion

%

Location 1

MGA Select

70/106

66

99/106

93

CO-Synch + CIDR

67/104

64

99/104

95

Location 2

MGA Select

53/80

66

77/80

96c

CO-Synch + CIDR

56/78

72

76/78

97c

Location 3

MGA Select

26/45

58

42/45

93

CO-Synch + CIDR

29/43

67

42/43

98

Location 4

MGA Select

52/96

54

87/96

91

CO-Synch + CIDR

62/98

63

91/98

93

Combined

MGA Select

201/327

61

305/327

93

CO-Synch + CIDR

214/323

66

308/323

95

a. Pregnancy rate to fixed-time AI determined by ultrasound 40 to 45 days after AI. b. Pregnancy rate at the end of the breeding season determined 50 to 60 days after the end of breeding season. c. Pregnancy rate at the after-45-days breeding season.